Use etag caching for merge request attributes which depends on the current user abilities

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The idea is to gradually migrate the fields out of MergeRequestPollWidgetEntity to the MergeRequestPollCachedWidgetEntity. The following attributes depends on the current user abilities:

Frondend attribute	Backend attribute	Why becomes outdated
canMerge	merge_path	attr individual to a user
canRevertInCurrentMR	can_revert_on_current_merge_request	attr individual to a user
canCherryPickInCurrentMR	can_cherry_pick_on_current_merge_request	attr individual to a user
canRemoveSourceBranch	can_remove_source_branch	attr individual to a user
canPushToSourceBranch	can_push_to_source_branch	attr individual to a user
removeWIPPath	remove_wip_path	attr individual to a user
commitMessage	default_merge_commit_message	by changing MR attribute, but issues parsing contains a check specific to a user
commitMessageWithDescription	default_merge_commit_message_with_description	by changing MR attribute, but issues parsing contains a check specific to a user
newBlobPath	new_blob_path	attr individual to a user
canCancelAutomaticMerge	cancel_auto_merge_path	attr individual to a user
setToAutoMergeBy	merge_user	attr individual to a user

Related issue: gitlab-foss#61559 (closed)